UPDATE:

Today they're saying that they've received "credible information from British intelligence" that the brother had a family feud over what the dead man inherited. They're implying that the brother paid a hitman.

Now call me silly, but if the brother wanted to take over his inheritance, surely the hitman would have been given full details of the family and instructions to take them all out. But the girls were left alive, and they'll inherit their father's money. Whatever, it seems clear that this is what they're going along with.